1. Field of the Invention
The present invention relates generally to the field of optical microscopy imaging which uses structured or selective illumination or excitation and, more specifically, to a method using a minimum number of selective excitation patterns optimized for imaging of DNA microparticles.
2. Description of the Related Art
Synthetic Aperture Optics (SAO) imaging refers to an optical imaging method in which a series of patterned or structured light patterns are used to illuminate the imaging target in order to achieve resolution beyond what is set by physical constraints of the imaging apparatus such as the lens and the camera. In SAO, an imaging target is selectively excited in order to detect the spatial frequency information of the target. Since there is a one-to-one relationship between the frequency (or Fourier) domain and the object (or target) domain, SAO can reconstruct the original imaging target by obtaining its spatial frequency information.
FIG. 1A illustrates a conventional SAO method, and FIG. 1B illustrates a conventional SAO system. Referring to FIGS. 1A and 1B together, in conventional SAO, selective excitation (or illumination) 104 is applied to an imaging target 102, and the light scattered or fluoresced from the imaging target 102 is captured by optical imaging 106. The imaging target 102 can be composed of micro-particles in a randomly or regularly distributed pattern. Selective excitation (or illumination) 104 may be applied to the imaging target 102 by an illumination apparatus (not shown in FIGS. 1A and 1B) that is configured to cause interference 122 of two or more light beams 131, 132 on the imaging target 102. The excitation is selective or patterned, unlike uniform illumination used in conventional optical imaging techniques. For example, two beams 131, 132 may overlay or interfere on an imaging-target plane 102 to produce a two-dimensional (2D) sinusoidal excitation pattern.
FIG. 1C illustrates an example of a selective excitation pattern in the spatial domain and the frequency domain. Referring to FIGS. 1B and 1C, the exemplary selective excitation pattern 140 in the spatial domain is generated by interference of two beams 131, 132 on the imaging-target plane 102, resulting in a 2D sinusoidal excitation pattern. The angle (φ) between the two beams 131, 132 determines the pitch 143 of the pattern, which represents the spacing or periodicity of 2D sinusoidal fringe pattern 140. More specifically, the pitch 143 is substantially inversely proportional to sin(φ). The orientation φ of the pattern represents the amount of angular rotation of the 2D sinusoidal fringes 140 compared to its reference pattern, which in this example of FIG. 1C is shown as a 2D sinusoid comprised of vertical lines, although a different reference pattern such as a 2D sinusoid comprised of horizontal lines can also be used as the reference pattern. In mathematical terms, the orientation φ can be described as follows: if u is the normal vector of the plane formed by the two beams 131, 132 and if the projected vector of u on the imaging plane 102 is called v, then the orientation φ of the sinusoidal pattern 140 is the angular orientation of the vector v with respect to the frame of reference. The “phase” of the pattern is the periodic position of the 2D sinusoid with respect to the frame of reference. The range of the phases of the 2D sinusoid excitation pattern will be a value between 0 and 2π. The different phases can be obtained by changing optical path length of one beam.
As shown in FIG. 1C, the 2D sinusoid excitation pattern in the spatial domain can be shown as a conjugate pair ki, ki′ in the corresponding frequency domain (k-space). Each conjugate pair in the k-space corresponds to the pitch 143 and orientation φ of the corresponding 2D sinusoid pattern. The pitch 143 of the 2D sinusoid pattern 140 is determined by the radial distance r of the k-space point—more precisely, the pitch 143 is substantially the inverse of the radial distance r in the frequency domain. The orientation φ is the angle φ of the k-space points in a radial coordinate system in the frequency domain. Thus, a number of different excitation patterns may be generated by changing the pitch 143 of the 2D sinusoid pattern (or the angle (φ) between the two beams 131, 132) and changing the orientation φ of the 2D sinusoid pattern, with each different pair of pitch 143 and orientation φ of the 2D sinusoid pattern in the spatial domain corresponding to a different conjugate pair (radial distance r and orientation φ) in the k-space (frequency) domain.
Referring back to FIGS. 1A and 1B, the excited target 102 emits signals (or photons), and the signals are captured in optical imaging system 106 including an objective lens 124 and an imaging sensor (or imager) 126. The emitted signal will have a wavelength λE. The objective lens has magnification (Mag) and a numerical aperture NA=n×sin θ, where n is the index of refraction of the medium in which the lens 124 is placed and θ is the half-angle of the maximum cone of light that can enter or exit the lens 124. Typically, the imaging sensor 126 can be a charge-coupled device (CCD), complementary metal-oxide-semiconductor (CMOS) image sensor, or any other photon detectors in a matrix or array format including a plurality of pixels m. Note that, in some applications, the emitted signals from the target 102 may be directly captured by the imager 126 without going through the objective lens 124.
Then, it is determined 108 whether the images corresponding to all the phases of the 2D sinusoid excitation pattern were obtained. If images corresponding to all the phases of the 2D sinusoid excitation pattern were not obtained in step 108, the excitation phase is changed 114 and steps 104, 106, 108 are repeated for the changed excitation phase. If images corresponding to all the phases of the 2D sinusoid excitation pattern were obtained in step 108, then it is determined 110 whether the images corresponding to all the 2D sinusoid selective excitation patterns were obtained. If images corresponding to all the 2D sinusoid selective excitation patterns were not obtained in step 110, the excitation pattern is changed by using a different spatial frequency (e.g., changing the pitch 143 and the orientation φ of the 2D sinusoid pattern) and steps 104, 106, 108, 114 are repeated for the next selective excitation pattern.
If images corresponding to all the 2D sinusoid selective excitation patterns were obtained in step 110, then finally the captured images are sent to a computer for SAO post processing 112 and visualization. In conventional imaging, the resolution of the SAO imaging system is determined by the numerical aperture NA of the lens 124, the wavelength λE of the emitted light, and the pixel size. In contrast, in SAO imaging, the resolution of the imaging system is beyond what can be achieved by the numerical aperture NA of the lens 124, the wavelength λE of the emitted light, and the pixel size. Thus, as shown in FIG. 1B, the images captured through steps 104, 106, 108 of FIG. 1A are raw images RIi, with a resolution lower than (insufficient for) the resolution needed to resolve the objects on the imaging target 102. However, multiple sets of the lower resolution raw images RI, are captured for different excitation phases and spatial frequencies (excitation patterns) to obtain the complete raw image set 128, which then goes through SAO post-processing 112 to synthesize the final image FI that has a resolution higher than the resolution of the raw images RIi. The resolution of the final image FI obtained by SAO post-processing is sufficient for resolution of the objects on the imaging target 102. The methodology for SAO post-processing 112 for synthesizing high resolution images FI from lower resolution raw images RI, is well known. Raw images RIi are converted into k-space information of the high resolution images FI, and this information is Fourier transformed to synthesize or reconstruct the high resolution images FI. For example, one example of the SAO post-processing methodology can be found in U.S. Pat. No. 6,016,196, issued on Jan. 18, 2000 to Mermelstein, entitled “Multiple Beam Pair Optical Imaging,” which is incorporated by reference herein.
Applying SAO to DNA (deoxyribonucleic acid) or RNA (ribonucleic acid) sequencing presents a number of challenges. The term “nucleic acid” herein includes both DNA and RNA. In DNA or RNA sequencing, single molecule or amplified clones of a DNA template (collectively referred to as “microparticle”) are immobilized onto a planar substrate. The array of microparticles then goes through multiple cycles of chemical reaction and optical detection. FIGS. 2A, 2B, and 2C illustrate different types of individual sequencing microparticles that can be used for DNA sequencing. FIG. 2A illustrates an individual microparticle 202 formed by a 1-micrometer diameter bead 208 covered with clonal DNA molecules 210 that have been previously amplified by a water-in-oil emulsion PCR technique. The bead 208 is attached directly to the substrate 204 in fluid 206. FIG. 2B illustrates an individual microparticle 202 as a cluster of clonal DNA molecules 210 attached to the substrate 204 and placed in fluid 206. The DNA molecules 210 have been previously amplified by a bridge amplification technique. FIG. 2C illustrates an individual microparticle as a single DNA molecule 210 attached to the substrate 204 and placed in fluid 206. The single DNA molecule 210 is sequenced without amplification.
The distribution of DNA microparticles can be random or regular. FIGS. 3A and 3B illustrate some examples of the distribution of DNA microparticles. If Δx is defined to be the spatial resolution of an imaging system (i.e., Δx is the minimum distance of two point objects that can be resolved by the imaging system), Δx is typically designed to be about half of the distance between adjacent microparticles 202 (see FIG. 3A). In DNA sequencing applications, it is highly desirable for an optical imaging system to achieve both high resolution and high scanning speed at the same time. SAO imaging is promising since it can image a large area using a low magnification lens and camera without sacrificing resolution. The resolution of SAO imaging is obtained from the high resolution illumination patterns and post-processing. However, SAO requires selective excitation to be repeated for a number of selective excitation patterns. Conventional SAO imaging uses a large number of SAO excitation patterns, often including many redundant or even irrelevant illumination patterns. The number of excitation patterns in conventional SAO is merely determined based on the hardware architecture of the illumination system, without regard to other factors. The large number of excitation patterns in conventional SAO makes it impractical for use in DNA sequencing, as conventional SAO does not offer the cost and throughput benefit in DNA sequencing compared to conventional optics. Also, conventional SAO hardware is large, complex, difficult to scale, and mechanically and thermally unstable, requiring large space and extremely careful control of temperature and mechanical vibration for continuous run, making it particularly impractical for use in DNA sequencing which requires repeated, continuous runs of SAO over a very large number of DNA microparticle arrays.